The general rule of thumb for MAME is to buy the fastest Intel CPU you can afford (within reason). Intel CPUs are quite a bit faster with most drivers, especially in 64-bit MAME.
With that said, if you don't mind running somewhat older versions of MAME, you can generally get away with slower CPUs. Still stick with Intel however. I'd say somewhere around a 2.5GHz Core2Duo as a minimum. Anything newer and/or faster is better, of course.